1. Field of the Invention
The present invention relates generally to programmable electronic digital thermostats for use with heating and cooling systems, and more particularly to an improved power supply circuit for programmmable electronic digital thermostats.
2. Description of the Prior Art
Known electronic digital thermostats provide substantial energy savings by controlling conventional temperature conditioning systems in accordance with selected temperatures for a sequence of time intervals. Examples of electronic digital thermostats are disclosed in U.S. Pat. Nos. 4,298,946, 4,388,592 and 4,442,972 which are assigned to the present assignee.
Advantageously operating power for electronic digital thermostats is derived from an external low voltage 24 volt AC (alternating current) power source from a conventional voltage step-down transformer of the furnace electrical system. Utilizing existing wiring for an electromechanical thermostat, such as a standard 4-wire connection scheme facilitates installation of programmable electronic digital thermostats. Also it is preferred that a backup battery is used to provide operating power in the event of a power interruption in the external low voltage 24-volt AC power source. Disadvantages of various known power supply arrangements used with programmable electronic digital thermostats are that they are generally complex and are costly in manufacture.
An improved power supply for electronic digital thermostats including the above advantages is taught by U.S. Pat. No. 4,799,176, issued Jan. 17, 1989 to the present inventor Joseph J. Cacciatore, and assigned to the present assignee. The power supply includes a bridge rectifier connected between the external AC power source and a controlled semiconductor switch for selectively activating or deactivating an external relay for controlling the operation of the temperature conditioning system. A voltage regultor is coupled at its input to the bridge rectifier for providing a predetermined operating voltage to the programmable thermostat. A capacitor is connected to the voltage regulator input for charging to a predetermined voltage potential. Control circuitry is connected between the capacitor and the controlled semiconductor switch for sensing the voltage potential of the capacitor and for deactivating the semiconductor switch in response to a sensed voltage potential below a second predetermined voltage potential to enable the capacitor to recharge to the first predetermined voltage potential without deactivating the external relay. While this disclosed power supply provides improvements over prior art power supply arrangements, it is desirable to provide a further improved power supply capable of reliably and effectively providing operating power for an electronic digital thermostat while eliminating the need for such control circuitry, and that is simple and inexpensive to make and that is not unnecessarily large in size.